The invention relates to an arrangement for the current limitation during the current supply of electrically excited gas lasers comprising a rotary current transformer, self-inductances provided in front of a rotary current bridge rectifier, and a capacitor, if desired.
With electrically excited gas lasers a glow-discharge must be maintained in the space within the optical resonator for the formation of a so-called plasma. Thus the operational voltage of such a laser is chosen to be of a value dependant upon the pressure of the gas to be excited, which gas may contain, for instance, Ar, Kr, Xe, F, N.sub.2 O, HF, N.sub.2, CO, CO.sub.2 and He, and the spacing between the electrodes; the higher the pressure of the gas and the greater the spacing between electrodes, the higher the value of the operational voltage.
If the electrodes of a gas laser are fed with a sufficiently high voltage, a gas discharge will ignite with a narrow break-through channel forming at first. If the current flowing in the break-through channel is increased so rapidly that the primary discharge channel has no time to widen laterally, high current densities occur there, leading to a very strong heating of the channel, with a discharge in the form of an electric arc thus finally forming. This type of discharge is not suited for the excitation of the laser gas, due to the high temperatures prevailing therein.
The field strength E in the discharge is of a decisive influence on the excitation of the gas molecules, for instance of CO.sub.2 molecules, since the gain of energy by the electrons between two collisions of gas molecules is proportional to the value E/p (p=pressure). With E/p==(0.075 V/Pa.multidot.cm), about 70% of the energy delivered by the electrons is used for the desired excitation of oscillations of the gas molecules, whereas, when observing this relation, only a small part of the electron energy serves for the excitation of the electron sheaths and for the ionization of the atoms, thus getting lost in view of the attainable optical output performance.
Yet, with pressures of above approximately 0.133 bar and when exceeding a critical value of the current intensity, a glow discharge becomes unstable and extremely easily contracts into an arc channel. However, as already mentioned, only a glow discharge is suited for excitation, due to the low gas temperature with a simultaneously high electron energy prevailing therein. The period of time necessary for a glow discharge to develop into an arc discharge is in the range of some microseconds (.mu.s), so that a spacially homogenous plasma with gas pressures higher than about 0.133 bar can be maintained for only few .mu.s (cf. e.g. W. W. Duley, CO.sub.2 -Lasers, Effects and Applications, Academic Press, New York, S. Francisco, London 1976, pp. 15 to 72, in particular pp. 36 to 39).
For preventing the development into an arc discharge, accordingly large ballast resistors are usually used, which delimit the current so that the discharge current cannot exceed a certain value with a predetermined supply voltage.
The greater the negative differential resistance of the glow discharge, the greater the ballast resistor must be in order to be able to adjust a stable operating point.
With relatively high pressures, multiple subdivided electrodes are used. Therein, discharge instabilities and inhomogenities occur, which again may be prevented by high ohmic ballast resistors at each partial electrode. The superposition of such resistors has the extremely great disadvantage that approximately as much electric power is used in the ballast resistors as in the gas discharge itself and is given off as heat loss. CO.sub.2 lasers of the above-defined kind including segmented anodes and air-cooled ballast resistor chains are described, for instance, in the pamphlet 2/79 (1979) of Kristalloptik-Laser-bau GmbH having the designations KEL T 51 and KEL T 52; the high-voltage supply there is effected by means of a rotary current transformer with a subsequently arranged rotary current bridge rectifier.
From U.S. Pat. No. 3,758,877 it has become known to actuate an electrically excited, continuously working CO.sub.2 gas laser with a current supply system comprising a current source for high frequency AC, a rectifier, a filter capacitor in parallel connection and an inductance in series connection. It is essential that the frequency of the AC exceeds a certain value, i.e. that limit which separates the region of the negative differential resistance of the laser discharge tube at low frequencies from that of the positive differential resistance. The indicated limit generally is in the magnitude of 1 kHz.
With such an arrangement a high-frequency AC has to be generated, which alone necessitates considerable expenditures. The entire arrangement has to be shielded off in order to prevent disturbances of the surrounding caused by high-frequency stray fields. For smoothing the pulsating AC generated, a capacitor also is required.
In German Offenlegungsschrift No. 26 57 893 a current supply in a pulsed mode is suggested for a gas discharge laser, in which a main DC supply is connected in series with a current-limited DC supply and the laser electrodes, and wherein diode means are connected via the second current supply in order to avoid the same and to feed voltage from the main current supply to the laser electrodes after the laser gas has been ionized. According to a preferred embodiment of this current supply system a rotary current transformer with a rotary current bridge rectifier is provided as second current supply in order to be able to do without capacitor, thus ensuring a short recovery time of the current supply. The chokes provided between the source for the three-phase AC and the primary coils apparently serve for damping the input currents to the current transformer, supporting the behaviour desired according to German Offenlegungsschrift No. 26 57 893 to reduce the voltage of the rotary current supply under load to practically zero, because the high primary current of the transformer in the chokes causes a considerable voltage drop and thus a substantially smaller primary voltage (p. 8 of German Offenlegungsschrift No. 26 57 893).